The present invention relates to a glass antenna disposed on a window glass for, e.g., a vehicle and, more specifically, to a glass antenna having two antenna conductor wire elements which are arranged in a defogger and are capacitively coupled to each other.
In general, as an antenna for a vehicle, a pole antenna, which is constituted by a pole (rod) projecting on the body of the vehicle in an insulating state, and feeds electric power to the pole, is popular. However, this pole antenna is easily bent or damaged, and generates wind noise upon travelling. For these reasons, a glass antenna has been put into a practical application in place of the pole antenna.
As disclosed in, e.g., Japanese Utility Model Laid-Open No. 63-92409, the glass antenna has an antenna wire disposed in the vicinity of a defogger arranged in a window glass of a vehicle, and feeds electric power to the wire.
However, in the conventional glass antenna, the reception performance of the antenna is tuned by disposing the antenna wire in the vicinity of the defogger, and an established method of improving the performance of the antenna is not available. Therefore, systematic tuning is not possible, and it is difficult to predict the tuning result. In addition, the arrangement of the antenna itself becomes complicated.
On the other hand, as disclosed in Japanese Patent Laid-Open No. 62-131606, another antenna system has been proposed. In this system, a transparent conductive film is formed on a glass surface, an antenna body having a feed point is disposed on the glass surface above the conductive film, and the antenna body is capacitively coupled to the transparent conductive film.
Also, in U.S. Pat. No. 5,029,308, a first antenna conductor wire is arranged in a region where defogger hot wires extend, and extends in the up-and-down direction at substantially the center of a defogger region, so that the first antenna conductor wire and the hot wires crossing the conductor wire are electrically connected to each other. Furthermore, a second antenna conductor wire is arranged on an upper region (or lower region) of the defogger to be connected to the uppermost (or lowermost) hot wire of the defogger. More specifically, the first and second antenna conductor wires serve as a single antenna. However, when the first and second antenna conductor wires are connected to each other, a DC current flowing through the defogger is shunted to the first antenna conductor wire, and the defogging effect is impaired near the connection points.
In view of this problem, in U.S. Pat. No. 5,029,308, a capacitor is arranged between the first and second antenna conductor wires, so that the current flowing through the defogger is not shunted. Note that the capacitance (or capacity) of this capacitor is selected to have a value, which does not have a high impedance with respect to the reception frequency band since the first and second antenna conductor wires must serve as a single antenna.
Furthermore, in an antenna system disclosed in Japanese Patent Laid-Open No. 55-60304, a first antenna conductor wire is arranged in the up-and-down direction in a defogger region, and a second antenna conductor wire is arranged outside the defogger region. A first conductive wire which is connected to the first antenna conductor wire and extends in a direction perpendicular to the first antenna conductor wire (e.g., parallel to defogger hot wires), and a second conductive wire connected to the second antenna conductor wire are arranged close to each other on a glass surface so as to be capacitively coupled to each other.
In the above-mentioned prior arts (Japanese Utility Model Laid-Open No. 63-92409 and Japanese Patent Laid-Open No. 62-131606), although the antenna body is capacitively coupled to the transparent conductive film, if a thin film is used to assure transparency of the conductive film, its electrical resistance value inevitably becomes very high, and a reception current becomes hard to flow. As a result, satisfactory antenna performance cannot be expected in a practical use.
In U.S. Pat. No. 5,029,308, since the capacitor is selected to have a low impedance in the frequency band of received radio waves, the defogger hot wires serve as an antenna, and hence, a heating current flowing through the hot wires influences the antenna, resulting in deteriorated antenna performance.
Also, in Japanese Patent Laid-Open No. 55-60304, since the shape of the antenna arranged outside the defogger region is not taken into consideration, in other words, since a measure for preventing the defogger hot wires from serving as an antenna is not taken, the antenna performance deteriorates as in U.S. Pat. No. 5,029,308.
As described above, with the conventional glass antenna, in order to minimize deterioration of the antenna performance and to prevent the defogger current from flowing through the antenna conductor wires, the antenna conductor wires are arranged at the center of the defogger, i.e., at the center of the glass. For this reason, the field of view is disturbed at near the center of the glass (in particular, near the center where the defogger hot wires extend), resulting in poor visibility.
Since the conventional glass antenna is arranged under the condition that some of antenna conductor wire elements are arranged on a region without the defogger, i.e., since the antenna conductor wire elements require a blank region without defogger hot wires, the defogging function is impaired in such a blank region, and rear visibility similarly deteriorates.